Abstract: Analysis of the large collection of genetically modified plants has shown that transgenes in a great of transgenic plants can’t be expressed as expected. In some cases, transgenes are inactive and the transgene expression couldnt be detected throughly, which are called as gene silencing. Transgene silencing has greatly hinderred the application of the technic of plant gene transformation. Various factors seem to play a role, including of DNA methylation, transgene copy number and the DNA region where transgenes inserted. Basically there are two kinds of gene silencing, transcriptional gene silencing (TGS) and post transcriptional gene silencing (PTGS). According to the study on a mount of transgenic plants, several possible mechanisms by which silencing is achieved have been presented, such as RdRP cRNA , RNA threshold and RdDM models. The more understandings to the mechanisms will largely speed the research on plant gene engineering.